Hmmm. Not sure why you couldnt copy and paste, let me go over the process and let me know if you still have any trouble.
Single (left) click the September COGS cell to select it (do not double click).
Hit CTRL-C
Single (left) click the October COGS cell to select it.
Hit CTRL-V
This should work because none of these cells should be locked, but let me know!
Thanks,
Scott